The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to an image forming apparatus of the type electrostatically transferring a latent image formed on a photoconductive drum or similar image carrier to a recording medium held by a clamp drum or similar medium conveying member.
Conventional image forming apparatuses included one which forms a latent image on a charged image carrier, electrostatically transfers the latent image to a recording medium retained by a clamp drum or similar medium conveying member, and then develops the latent image by a developing device. In this type of apparatus, when the image carrier is implemented by, for example, an organic photoconductor (OPC) extensively used in electrophotographic copiers, it is charged to a potential higher than -600 V. An exposing device exposes the charged surface of the image carrier imagewise so as to form a latent image thereon. When the image carrier is discharged and charged during the course of image formation, it is connected to ground since a predetermined voltage is applied thereto from an external power source. On the other hand, when the latent image is electrostatically transferred from the image carrier to a recording medium held by the medium conveying member, it is necessary to transfer the potential of the image carrier to the medium efficiently. For this purpose, the surface of the image carrier and the medium are brought into close contact, and a bias voltage is applied from an external power source. In the case of positive-to-positive image formation, the bias voltage is selected to be higher than about 400 V. While the bias voltage is applied, gaseous discharge is effected to transfer the latent image from the image carrier to the medium. In the event of such image transfer, the image carrier is held in close contact with the medium and driven at the same peripheral speed as the medium conveying member retaining the medium thereon. To drive the image carrier, the rotation of the conveying member may be transmitted to the image carrier by gears, or an exclusive drive source may be assigned to the image carrier.
However, the conventional image forming apparatus of the type described has various problems left unsolved, as enumerated below.
(1) When the medium and/or the image carrier has local defects, e.g., defective voltage resistivity, the voltage left for image transfer is lowered. As a result, defective image transfer extends to the entire area, not to speak of the local defects.
(2) The medium conveying member includes a conductive portion which contacts the rear (non-recording surface) of the medium for conduction with the electrode layer of the medium. When the image carrier is connected to ground, a transfer bias is usually applied to the conductive portion in order to electrically insulate the medium conveying member from the apparatus body. Hence, it is necessary to electrically insulate the medium conveying member from the apparatus body or to connect the conveying member to a drive line via an insulating material. However, an insulating material is difficult to machine with accuracy and is susceptible to wear and changes in temperature and humidity, obstructing precision drive, indispensable for high definition images. Particularly, when the medium conveying member is implemented as a drum, the drum should be provided with a diameter matching the size of the medium and, therefore, a relatively large diameter. This kind of medium conveying member magnifies even unnoticeable changes around the axis of rotation thereof, impairing the register of images.
(3) When the image carrier is driven, via gears, as stated previously, gear marks appear due to back lash and other similar causes. This is particularly true in the case of high density image writing. Further, the distance between the gear of the image carrier and that of the drive line unavoidably changes, so that the pitch circle of the gears cannot be maintained constant. As a result, the image carrier and the conveying member move at different speeds at their contacting portion. On the other hand, when an exclusive drive source is assigned to the medium conveying member, small differences in speed between the image carrier and the conveying means are sequentially accumulated as the medium is transported, resulting in a noticeable dislocation of an image. This problem is especially serious when the medium has a substantial length.
(4) In the conventional electrostatic image transfer, the effective transfer voltage is undesirably high. Here, the term "effective transfer voltage" refers to a gap voltage calculated on the assumption that the electrostatic capacity of the gap between the surface of the image carrier and the recording surface of the medium is infinite. Therefore, regarding the calculation of a potential distribution, the electrostatic capacities of the image carrier and medium are assumed to be negligible. The substantial effective transfer voltage is produced by adding the charge potentials of the medium and image carrier before image transfer and the bias voltage for image transfer, taking account of polarity and grounding portion. The charge potential of the image carrier is the potential measured on the surface by using the base of the image carrier as a reference. In the conventional arrangement, the charge potential of the image carrier is higher than 600 V, the bias voltage for image transfer is 400 V, and the charge potential of the medium is 0 V. The sum of these voltages, i.e., the effective transfer voltage is higher than 1000 V. When a latent image is transferred by such a high effective transfer voltage, white dots ascribable to abnormal discharge appear in a solid image.